If you go the classes route to learn about making your own custom garments (which you totally should if you really want to do more as a cosplayer), I recommend taking sewing classes first, and then get into patternmaking. If you go into pattern drafting with that foundation of sewing knowledge, you'll have a better understanding of what you need to do to really complete a workable pattern.

While you could do patternmaking first before sewing classes, the people I've seen go that route tend to struggle a lot more, make more mistakes, and don't really have as much grasp of the finishing additions needed for when the garment will actually be sewn.